(730e) Kinetics of Hydrocarbon Steam Reforming over Mn-Cr-O Spinel Oxides

Steam cracking of hydrocarbons is an important route to produce olefins, but coke formation significantly impedes the economy of the process. In particular, the buildup of coke in the reactor tubes increases pressure drop and decreases heat transfer in the cracker. The reactor tubes composed of Fe-Cr-Ni metal alloys form a surface barrier layer of MnCr₂O₄-Cr₂O₃ upon oxidative pretreatment, but little is known about the reactivity of this barrier layer. In this work, we have synthesized and characterized a series of Mn-Cr-O spinel oxides with different stoichiometry (Mn_xCr_3-xO₄, x = 0.5, 1.0, 1.5). Their catalytic performance in steam reforming of various hydrocarbons was evaluated and compared to pure oxides.

The excess Cr existed as Cr₂O₃ in the Cr-rich Mn_0.5Cr_2.5O₄ sample, whereas no crystalline Mn oxide phase was detected in the Mn-rich Mn_1.5Cr_1.5O₄ sample. Results from XPS and EXAFS suggest the excess Mn exists as Mn³⁺, which resides in the octahedral sites of the spinel lattice as a substitution for Cr³⁺. While Mn₃O₄ underwent in situ reduction to MnO during reaction, the Mn-Cr-O spinel catalysts were structurally stable and active for both ethylene steam reforming and toluene steam reforming. The reforming rate was first order in hydrocarbon and nearly zero order in steam in both cases. Toluene steam reforming was strongly inhibited by excess H₂ (nearly negative first order) whereas no appreciable effect of excess H₂ was observed during ethylene steam reforming. The apparent activation energy of toluene steam reforming was substantially higher compare to that of ethylene steam reforming over all three spinel catalysts. The rate of propylene and benzene steam reforming was also investigated over the MnCr₂O₄ catalyst and was comparable to that of ethylene and toluene steam reforming under similar conditions.